Surface mining is the predominant exploitation procedure worldwide. Approximately, more than 90% of all non-metallic minerals and metallic minerals and more than 60% of coal are mined by surface methods. Over 30 billion tonnes of ore and waste materials that are mined each year and surface mining accounts for nearly 25 billion tonnes. The subsurface of the earth is the only source for fossil energy and mineral products, and mining is the only way to get at them (Adapted from Ramani, 2012).
Extraction of mineral or energy resources by operations exclusively involving personnel working on the surface without provision of manned underground operations is referred to as surface mining. While an opening may sometimes be constructed below the surface and limited underground development may occasionally be required, this type of mining is essentially surface-based. Traditional surface mining can be classified into two groups on the basis of the method of extraction: Mechanical extraction or aqueous extraction. The primary differences between these mining methods are the location of the ore body and the mode of mechanical extraction. (Adapted from Yamatomi and Okubo, 2009; Hartman, 2002 and Sharma, 2011).
Mechanical surface mining methods Includes:
- Open-pit mining: Open-pit mining is employed to remove hard rock ore (mostly metallic ore) that is disseminated and/or located in deep seams and is typically limited to extraction by shovel and truck equipment. Many metals are mined by the open pit technique: gold, silver and copper, to name a few.
- Quarrying: It is a term used to describe a specialized open-pit mining technique wherein solid rock with a high degree of consolidation and density is extracted from localized deposits. Quarried materials are either crushed and broken to produce aggregate or building stone, such as dolomite and limestone, or combined with other chemicals to produce cement and lime. Construction materials are produced from quarries located in close proximity to the site of material in order to reduce transportation costs. Dimension stone such as flagstone, granite, limestone, marble, sandstone and slate represent a second class of quarried materials. Dimension stone quarries are found in areas having the desired mineral characteristics which may or may not be geographically remote and require transportation to user markets.
- Strip mining: “Open-cast mining” techniques relate to the extraction of ore bodies that are near the surface and relatively flat or tabular in nature and mineral seams. It uses a variety of different types of equipment including shovels, trucks, drag lines, bucket wheel excavators and scrapers. Most strip mines process non-hard rock deposits. Coal is the most common commodity that is strip mined from surface seams.
- Auger mining: It is a surface mining technique used to recover additional coal from a seam located behind a highwall produced either by stripping or open-pit mining. Auger mining is especially employed when contour strip mining has been exhausted and the removal of overburden to access additional coal no longer becomes economically feasible.
Aqueous surface mining methods can be subdivided in Placer and Solution and Includes:
- Placer: concentrations of metals such as gold, titanium, silver, tin and tungsten are washed from within an alluvial deposit.
- Hydraulic mining: “Hydraulicking” applies high pressure water spray to excavate loosely consolidated or unconsolidated material into a slurry for processing. Hydraulic methods are applied primarily to metal and aggregate stone deposits, although coal, sandstone and metal mill tailings are also amenable to this method. Water supply and pressure, ground slope gradient for runoff, distance from the mine face to the processing facilities, degree of consolidation of the mineable material and the availability of waste disposal areas are all primary considerations in the development of a hydraulic mining operation.
- Dredging: When hydraulic mining occurs underwater it is referred to as dredging. In this method a floating processing station extracts loose deposits such as clay, silt, sand, gravel and any associated minerals using a bucket line, dragline and/or submerged water jets. The mined material is transported hydraulically or mechanically to a washing station which may be part of the dredging rig or physically separate with subsequent processing steps to segregate and complete processing. While dredging is used to extract commercial minerals and aggregate stone, it is best known as a technique used to clear and deepen water channels and floodplains.
- Solution: It is applied to the process of removing a soluble mineral by dissolving it and leaching it out.
- Surface Techniques and In-situ Leaching: Both are applicable to deposit of minerals that can be recovered usually by dissolution, but also by melting, leaching, or slurrying. The two methods are similar and are differentiated primarily through the location and the type of minerals recovered. Surface leaching generally employs heap (or dump) leaching of mineral values; copper, gold, silver, and uranium are common examples. In-situ mining specifically uses barren solution, introduced down by a set of wells. The loaded solution then returns to the surface through concentric or other set of wells. Chemicals and/or bacteriological reagents usually are mixed with water in order to selectively dissolve the valuable minerals.
- Evaporite (or salt) Mining: In solution mining, fresh water is injected through a pipe into deep shafts that end in the salt beds, and salty water (brine) is drawn upward and dried, to recrystallize the salt. Or, salty brine found in shallow wells can simply be pumped to the surface and dried there, to make salt.
Mining conditions are likely to be more difficult in the future. While finding a world class mineral deposit cannot be ruled out, if history is a guide, future discoveries will be deeper, thinner, lower grade and have severe conditions, all increasing the difficulty of mining and processing. Therefore, operators should continuously re-evaluate traditional surface mining methods, incorporating new technologies and practices. Hybrid and new methods will be required for delivering economic results and efficiency when approaching these new mineral deposits (Adapted from Ramani, 2012).
How can autonomous equipment and vehicles be applied to mining? Are there opportunities to enhance this application by the use of telemetry and artificial intelligence?
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Hartman, H. L. and Mutmansky, J. M. “Introductory Mining Engineering”, Published by Wiley, Hoboken, NJ, USA, (August, 2002).
Okubo S. and Yamatomi, “Underground Mining Methods and Equipment, Civil Engineering- Vol. II, Encyclopedia of Life Support System (EOLSS), EOLSS Publishers/UNESCO (2009).
Sharma, P. D., “Coal and Metal (Surface and Underground) Mining – An Overview”, Weblog of Partha Das Sharma. (2011). Last accessed on 07/23/2016 at https://miningandblasting.files.wordpress.com/2009/09/coal_and_metal_surface_and_underground_mining_an_overview.pdf
Dunbar, W. S., “Basics of Mining and Mineral Processing”, Americas School of Mines, University of British Columbia, PWC. (2012). Last accessed on 07/23/2016 at https://www.pwc.com/gx/en/mining/school-of-mines/2012/pwc-basics-of-mining-2-som-mining-methods.pdf
RitchieWiki Team, “Underground Mining”, Ritchie Bros. Auctioneers (2012). Last accessed on 07/23/2016 at https://www.pwc.com/gx/en/mining/school-of-mines/2012/pwc-basics-of-mining-2-som-mining-methods.pdf
Feature: Bingham Canyon copper mine, UT, USA: Rio Tinto, Kennecott Utah Copper Corp. Author: Spencer Musick.
Figure 1: A surface coal mine in Gillette, Wyoming. Author: Greg Goebel
Figure 2: Limestone Quarry. Author: Mike Pierce
Figure 3: Dredge no. 3, working in the Klondike River, May the 31st, 1915. Author: Frank and Frances Carpenter collection
Figure 4: Cyanide leaching “heap” at a gold mining operation near Elko, Nevada. Author: U.S. Fish and Wildlife Service
Figure 5: Hydromonitor – Hydraulic mining. Author: Paesslergung